1. Field of the Invention
The present invention relates to an image processor and image processing method that process multi-value image data that corresponds to the same area in order to print an image in the same area by relatively moving a printing unit a plurality of times or by relatively moving a plurality of printing element groups with respect to the same area of the printing medium.
2. Description of the Related Art
In inkjet printing apparatuses, a multipass printing method that completes an image in the same area of a printing medium by performing a plurality of printing scans by a printing head over that same area is a technique for reducing density unevenness and stripes in the printed image. However, even by adopting the multipass printing method, deviation of the dot printing position may occur between that of a prior printing scan and that of a later printing scan due to fluctuation in the amount the printing medium is conveyed. This kind of deviation causes fluctuation in the dot coverage rate, which causes defects in the image such as density fluctuation and density unevenness.
This method is a technique for reducing these kinds of image defects, in which image data are divided into divisions that correspond to different printing scans in the stage of the multi-value image data before binarization, and then each of the multi-value image data after the division is binarized independently (with no correlation) (see Japanese Patent Laid-open No. 2000-103088). FIG. 10A is a diagram that illustrates the state of the arrangement of dots that are printed based on image data that were processed by the method disclosed in Japanese Patent Laid-open No. 2000-103088. In the diagram, the black dots 1501 are dots that are printed in the first printing scan, the white dots 1502 are dots that are printed in the second printing scan, and the gray dots 1503 are overlapping dots that are printed in the first printing scan and second printing scan.
With this kind of dot arrangement, even though the dot group that is printed in the first printing scan and the dot group that is printed in the second printing scan shift in the main scanning direction or sub scanning direction, the dot coverage rate with respect to the printing medium does not fluctuate much. The reason for that is that areas where dots that are printed in the first printing scan and dots that are printed in the second printing scan overlap newly appear, however; there are also areas that exist where two dots that were originally to be printed such that they overlap no longer overlap.
However, in the method disclosed in Japanese Patent Laid-open No. 2000-103088, binary data are not correlated among a plurality of planes, so graininess may become worse. For example, from the aspect of reducing graininess, the ideal in highlighted areas would be to evenly disperse the dots while maintaining a set distance between a few dots. However, in a configuration in which binary data are not correlated among a plurality of planes, the locations of overlapping dots (1603) and locations of dots printed adjacent to each other (1601, 1602) occur irregularly as illustrated in FIG. 10C, and an accumulation of these dots cause the graininess to become worse. In other words, when the dispersion of the dots is increased in order to suppress the graininess (keep the dot overlap rate low), unevenness occurs as the density changes, and when the dot overlap rate is increased in order to suppress the unevenness due to this density change, the graininess becomes worse.
Therefore, the inventors diligently studied a method for solving both of these two problems simultaneously, and as a result gained the following knowledge. In other words, both the density change and graininess described above respectively have certain allowable ranges (ranges in which they are hardly noticeable by human perception). Therefore, by controlling the dot overlap rate so as to keep both within the respective allowable ranges, output of an image in which defects do not stand out can be expected. More specifically, the allowable ranges described above change depending on the type of a printing medium, the type of image (whether the image is text or a photograph), or image quality required for a mode a user has set, and the suitable dot overlap rate is not necessarily always constant. Therefore, preferably, in a system in which a plurality of printing modes respectively having different printing speeds or multipass numbers depending on various conditions are prepared, the dot overlap rate is adjusted according to the various conditions shown above.